gosh, so I am starting to get a little stressed about this....I kinda thought the Dad would stay at home if he wasn't working and look after the other siblings if there were any. I have hired a Fairy to do games and face painting for an hour and a half - if we go over a certain number of kids the price goes up - but would feel mean saying no that the siblings couldn't have their face painted, etc, join in, do the parents usually let you know how many will be attending or just rock up on the day? This is turning out bigger than I thought....
... Nah don't let it stress you this to be a day for you look forward to also to watch your little one super happy at their own b'day party with their little friends, you don't want the extra hassle of stress leading up to it !!

Why not put the invitations in a colourful envelope & pop the one child's name on the front in again in different colourful texta to make it look fun in capital letters to stand out and then maybe put in brackets in lower case writing something like this for example * CHARLIE (and a parent) are invited to BOOBOO's 4th birthday party *... so then it's hopefully clear that you are inviting one child and one parent without looking offensive !!

Just as JaZaMum suggested i too always have at least two or three 'extra' lolly/loot bags just incase for any unexpected guests (which to be honest i so far haven't had happen)